In communication networks which applies LTE (Long Term Evolution) based radio access technologies, accurate time and phase alignment of the internal clock is important. Time and phase synchronisation is required for LTE-TDD (Time Division Duplex) many LTE-FDD (Frequency Division Duplex) coordination features e.g. for joint transmission, a wireless communication device receives data from multiple cells or multiple RBSs (Radio Base Stations), which offers better performance, but puts harder requirements on synchronisation. In packet synchronisation networks, a major problem for synchronisation protocols is the variance in the send time, access time, propagation time, and the receive time.
From the infrastructure perspective, mobile operators have a broad range of topologies to support. The physical network using different technologies such as microwave, fibre and copper wire will enable/limit different capabilities and characteristics. These differences in physical transport and in the different types of topologies, creates delay and delay variation that is unpredictable.
One solution for synchronising internal clocks in communication network is to distribute PTP (Precision Time Protocol) messages from a Grandmaster entity, which in generally is located centralised in the communication network, to PTP-clients at each cell site. The PTP protocol distributes PTP messages from a Grandmaster entity to transport network nodes and access network nodes who update their internal clocks based on the received time information in order to stay synchronised. A PTP system is a distributed, networked system consisting of a combination of PTP and non-PTP devices. PTP systems include a grandmaster entity, boundary clock nodes, ordinary clock nodes, and transparent clock nodes. The grandmaster entity is a form of synchronisation master node. Often the Grandmaster entity is located in a centralized part of the network; causing PTP messages to travel multiple hops. A “boundary clock” has multiple network connections and can accurately bridge synchronisation from one network segment to another. A synchronisation master is selected for each of the network segments in the system. The root timing reference is called the Grandmaster clock. The Grandmaster entity transmits synchronisation information to the clocks that are in its network segment. The boundary clocks with a presence on that segment then relay accurate time to the other segments to which they are equally connected. The transparent clock modifies PTP messages by including appropriate timestamps as they pass through the device. The Timestamps in the PTP messages are compensated for time spent traversing the network and equipment e.g. (switch/router).
With reference to FIG. 1, which is a schematic overview, a scenario of a communication network will now be described according to one example.
The communication network comprises a transport network with a plurality of transport network nodes 200, e.g. suitable switches, routers or gateways. In the FIG. 1, is further illustrated, two synchronisation master nodes 230, and three radio base stations 220, 222, 224 of a suitable radio access technology, Within this disclosure, as well synchronisation master nodes 230, and radio base stations 220, 222, 224 will be referred to as transport network nodes too. In the figure a mobile telephone 242 is illustrated which communicates via an access network with the radio base stations 222, 224, in accordance with any suitable radio access technology, e.g. LTE (Long Term Evolution), LTE-Evolution, 5G (of 3GPP (Third Generation Partnership Program)), UMTS (Universal Mobile Technology System), or HSPA (High Speed Packet Access).
The synchronisation master nodes 230 produces timing references, i.e. synchronisation references which are delivered by synchronisation packets to the radio base stations 222, 224 along respective paths (marked with dash-dotted lines and dotted lines, respectively). In the figure is also another mobile telephone 240 shown, which is served by the radio base stations 220, 222.
In practice, Radio Access Networks and transport networks are offered by different vendors and or organisations which have different requirements on their respective networks.
It is desired to enable network operators to make better use of installed communication resources, e.g. in order to serve end-users more appropriately. For instance it would be desired to decrease synchronisation time inaccuracy in order implement fast communication technologies.